THE acquittal of accused murderer Amanda Knox was full of "deficiencies, contradictions and illogical" conclusions Italy's high court has ruled, ordering a fresh look at all the evidence to determine whether she did kill her flatmate Meredith Kercher.

In March, the Court of Cassation overturned Knox's acquittal in the 2007 murder of British student Ms Kercher and ordered a new trial. The court has now issued its written reasoning.

Ms Kercher's body was found in November 2007 in her bedroom of the house she shared with Knox in Perugia.

Knox, now 25, and her Italian ex-boyfriend, Raffaele Sollecito, 29, were initially convicted and sentenced to lengthy prison terms, but a Perugia appeals court acquitted them in 2011, criticising virtually the entire case mounted by prosecutors. The appellate court noted that the murder weapon was never found, said that DNA tests were faulty and that prosecutors provided no murder motive.

A man from Ivory Coast, Rudy Guede, was convicted of the mruder in a separate hearing and is serving a 16-year sentence.

In the 74-page Cassation ruling, the high court judges said they "had to recognize that he (Guede) was not the sole author" of the crime. The judges though said he was the "main protagonist."

They said the new appeal process would serve to "not only demonstrate the presence of the two suspects in the place of the crime, but to possibly outline the subjective position of Guede's accomplices." It said hypotheses ran from a simple case of forced sex involving Kercher "to a group erotic game that blew up and got out of control."

The high court faulted the Perugia appeals court for "multiple instances of deficiencies, contradictions and illogical" conclusions. The new court must conduct a full examination of evidence to resolve the ambiguities, it said.

Knox left Italy a free woman after her 2011 acquittal, after serving nearly four years of a 26-year prison sentence. Now a student in Seattle, she has called the reversal by the Cassation "painful" but said she was confident she would be exonerated. Italian law cannot compel her to return for the new trial and her lawyers have said she has no plans to do so.

No date for the new trial has been set. Florence's appeals court was chosen since Perugia only has one appellate court.